1. Field of the Invention
The present invention relates to battery chargers and, more specifically, to a rapid battery charger.
2. Description of the Prior Art
Battery chargers for lead-acid batteries typically apply a charge current to a battery cell or cells under charge. The charge current drives an electro-chemical reaction that causes lead ions to precipitate out of an electrolytic solution onto a metal plate. Once a maximum amount of lead is removed from the solution, the battery is said to be "fully charged."
FIG. 1 shows a graph of a typical charging cycle for a battery. The x-axis 106 represents time and the y-axis represents both voltage 102 and current 104. The charge cycle comprises an application of a charging current pulse 120, a rest period 114 and the application of a depolarization pulse 116. Several consecutive charge cycles may be employed to charge a battery.
Initially, when a current (represented by a dotted line on the graph) is applied to a fully discharged battery the voltage (represented by a solid line) across the battery terminals is low, but increases at a very high rate relative to time 110. Eventually, the voltage levels off 112 and the rate of change, relative to time, approaches zero. At this point the efficiency of the charger is very low. This is due to the formation of a layer of lead-poor electrolyte forming around the metal plate to which the lead ions are precipitating. This layer is referred to as the "Helmholz" layer. As the Helmholz layer thickens, the rate of lead precipitation decreases. While one could apply a higher voltage to the charging current to increase precipitation, such a voltage would give rise to an increased level of heat production in the battery and, thus, a higher level of electrolyte evaporation.
The depolarization pulse 116 is applied to break up the Helmholz layer. Generally, the depolarization pulse 116 is a short pulse that forces the lead-poor electrolyte molecules away from the metal plate to which the lead is precipitating. Once the Helmholz layer is dissipated, normal charge current may again be applied with higher efficiency.
Many prior art chargers apply charging current pulses of fixed duration. During a portion 118 of the charging period 120, characterized by a rate of voltage change relative to time approaching zero, the Helmholz layer is relatively thick and less efficient charging is taking place. During this portion 118, most of the energy being applied to the battery is wasted in the form of heat.
Therefore, there is a need for a battery charger that senses when the rate at which voltage changes relative to time is near zero and that ceases to apply current near that time.